Glycoproteins are proteins that have carbohydrates (sugar molecules) covalently attached to their polypeptide chains. The carbohydrate portion of a glycoprotein can consist of single or multiple sugar units, which can be either simple sugars (monosaccharides) or complex sugars (oligosaccharides). The process of attaching carbohydrates to proteins is known as glycosylation, which occurs in the endoplasmic reticulum and Golgi apparatus within cells.
Glycoproteins play a wide variety of roles in biological processes, including:
- Structural support: Some glycoproteins, such as collagen, provide structural support to tissues by forming a part of the extracellular matrix.
- Cell adhesion: Glycoproteins like fibronectin and laminin are involved in cell adhesion, helping cells attach to one another and to the extracellular matrix.
- Cell signaling: Glycoproteins on cell surfaces can act as receptors, binding to signaling molecules and initiating various cellular responses. For example, many growth factor receptors are glycoproteins.
- Immune response: Glycoproteins play crucial roles in the immune system. Antibodies, also known as immunoglobulins, are glycoproteins that help recognize and neutralize foreign substances. Some cytokines, which are signaling molecules involved in immune response regulation, are also glycoproteins.
- Cell recognition: Glycoproteins on the surface of cells can serve as markers for cell recognition, allowing cells to identify and interact with each other. The ABO blood group antigens are an example of such glycoproteins.
- Lubrication and protection: Mucins are glycoproteins found in mucus, which help lubricate and protect the lining of various body cavities and surfaces.
- Protein stabilization: The addition of carbohydrates to proteins can stabilize their structures and protect them from degradation by proteases.
- Transport: Some glycoproteins, like transferrin, are involved in the transport of molecules within the body.
These examples illustrate the diverse functions of glycoproteins in living organisms. Alterations in glycosylation can lead to various diseases and disorders, including immune deficiencies, cancer, and genetic disorders like congenital disorders of glycosylation (CDG).